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EM6151V53SO8A Datasheet(PDF) 6 Page - EM Microelectronic - MARIN SA |
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EM6151V53SO8A Datasheet(HTML) 6 Page - EM Microelectronic - MARIN SA |
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6 / 10 page  R EM6151 Copyright © 2005, EM Microelectronic-Marin SA 6 www.emmicroelectronic.com rev. B / 06.06 Functional Description VIN Monitoring The power-on reset and the power-down reset are generated as a response to the external voltage level applied on the VIN input. The threshold voltage at which reset is asserted or released (VRESET) is determined by the external voltage divider between VDD and VSS, as shown on Fig. 8. A part of VDD is compared to the internal voltage reference. To determine the values of the divider, the leakage current at VIN must be taken into account as well as the current consumption of the divider itself. Low resistor values will need more current, but high resistor values will make the reset threshold less accurate at high temperature, due to a possible leakage current at the VIN input. The sum of the two resistors (R1 + R2) should stay below 500 kΩ. The formula is: VRESET = VREF x (1 + R1/R2). Example: choosing R1 = 200 kΩ and R2 = 100 kΩ gives VRESET =4.56 V (typical) for version V50 and V53. At power-up the reset output ( RES ) is held low (see Fig. 5). When VIN becomes greater than VREF, the RES output is held low for an additional power-on-reset (POR) delay TPOR (defined with the external resistor connected at ROSC pin). The TPOR delay prevents repeated toggling of RES even if VDD voltage drops out and recovers. The TPOR delay allows the microprocessor’s crystal oscillator time to start and stabilize and ensures correct recognition of the reset signal to the microprocessor. The RES output goes active low generating the power- down reset whenever VIN falls below VREF. The sensitivity or reaction time of the internal comparator to the voltage level on VIN is typically 3 μs. Timer Programming The on-chip oscillator allows the user to adjust the power-on reset (POR) delay TPOR and the watchdog time TWD by changing the resistor value of the external resistor ROSC connected between the pin ROSC and VSS (see Fig. 8). The closed and open window times (TCW and TOW) as well as the watchdog reset pulse width (TWDR), which are TTCL dependent, will vary accordingly. The watchdog time TWD can be obtained with figures 9 to 12 or with the Excel application EM6151ResCalc.xls available on EM website. TPOR is equal to TWD with the minimum and maximum tolerances increased by 1% (For Version 53, TPOR is one fourth of TWD). Note that the current consumption increases as the frequency increases. CAN-Bus Sleep Mode Detector (version 55) When the microcontroller goes into a standby mode, it implies that it does not send any pulses on the TCL input of the EM6151. After three reset pulse periods (TCW + TOW + TWDR) on the RES output, the circuit switches on an internal resistor of 1 M Ω, and it will have a reset pulse of typically 3 ms every 1 second on the RES output. When a TCL edge (rising or falling) appears on the TCL input or the power supply goes down and up, the circuit switches to the ROSC. Watchdog Timeout Period Description The watchdog timeout period is divided into two periods, a closed window period (TCW) and an open window period (TOW), see Fig. 4. If no pulse is applied on the TCL input during the open window period TOW, the RES output goes low for a time TWDR. When a pulse is applied on the TCL input, the cycle is restarted with a close window period. For example if TWD = TPOR = 100ms, TCW = 80 ms, TOW = 40ms and TWDR = 2.5ms. When VIN recovers after a drop below VREF, the pad RES is set low for the time TPOR during which any TCL activation is disabled. Timer Clearing and RES Action The watchdog circuit monitors the activity of the processor. If the user’s software does not send a pulse to the TCL input within the programmed open window timeout period a short watchdog RES pulse is generated which is equal to TWDR (see Fig. 6). With the open window constraint, new security is added to conventional watchdogs by monitoring both software cycle time and execution. Should software clear the watchdog too quickly (incorrect cycle time) or too slowly (incorrect execution) it will cause the system to be reset. If software is stuck in a loop which includes the routine to clear the watchdog then a conventional watchdog would not make a system reset even though the software is malfunctioning; the circuit would make a system reset because the watchdog would be cleared too quickly. If no TCL signal is applied before the closed and open windows expire, RES will start to generate square waves of period (TCW + TOW + TWDR). The watchdog will remain in this state until the next TCL falling edge appears during an open window, or until a fresh power-up sequence. The system enable output, EN , can be used to prevent critical control functions being activated in the event of the system going into this failure mode (see section “Enable- EN Output”). The RES output must be pulled up to VDD even if the output is not used by the system (see Fig 8). Combined Voltage and Timer Action The combination of voltage and timer actions is illustrated by the sequence of events shown in Fig. 6. On power-up, when the voltage at VIN reaches VREF, the power-on-reset, POR, delay is initialized and holds RES active for the time of the POR delay. A TCL pulse will have no effect until this power-on-reset delay is completed. When the risk exists that TCL temporarily floats, e.g. during TPOR, a pull-up to VDD is required on that pin. After the POR delay has elapsed, RES goes inactive and the watchdog timer starts acting. If no TCL pulse occurs, RES goes active low for a short time TWDR after each closed and open window period. A TCL pulse coming during the open window clears the watchdog timer. When the TCL pulse occurs too early (during the closed window), RES goes active and a new timeout sequence starts. A voltage drop below the VREF level for longer than typically 3 μs overrides the timer and immediately forces RES active and EN inactive. Any further TCL pulse has no effect until the next power-up sequence has completed. |
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